1. Technical Field
The present disclosure relates to methods for inter-carrier interference (ICI) cancellation or compensation, and more particularly, to two step ICI cancellation methods having low complexity and good performance in a time-varying channel, i.e., a mobile channel.
2. Discussion of the Related Art
Orthogonal frequency division multiplying (OFDM) is a robust technique for efficiently transmitting data over a channel. The technique uses a plurality of a sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to conventional frequency division multiplying (FDM) which can waste portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra, and thereby avoid inter-carrier interference (ICI).
In contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier.
The transmission of data through a channel using OFDM signals results in a tolerance to multipath delay spread and frequency selective fading, efficient spectrum usage, simplified sub-channel equalization, and good interference properties.
However, OFDM requires very accurate frequency synchronization between the receiver and the transmitter; with frequency deviation the sub-carriers are no longer orthogonal, causing ICI or cross talk between the sub-carriers.
Frequency offsets are typically caused by mismatched transmitter and receiver oscillators, or by a Doppler shift due to movement. While Doppler shift alone may be compensated for by the receiver, the situation is worsened when combined with multipath, as reflections will appear at various frequency offsets making them harder to correct.
This effect worsens as speed increases, and is a factor in limiting the use of the OFDM in high-speed vehicles such as in mobile TV applications in automobiles, buses, trains, etc.
There are several different standards for mobile TV using OFDM as an air interface, digital video broadcast handheld (DVB-H) quickly gaining ground in Europe, digital media broadcast (DMB) deployed in Korea, and forward link only (FLO), which offers mobile TV services in the U.S. via the media FLO system, for example.
For improved performance of an OFDM receiver, some of the sub-carriers in some of the OFDM symbols may carry pilot signals for measurement of the channel conditions, i.e. the equalizer gain and phase shift for each sub-carrier. Pilot signals and training symbols may also be used for time synchronization (to avoid inter-symbol interference, ISI) and frequency synchronization (to avoid inter-carrier interference, ICI, caused by Doppler shift).
FIG. 1 is a schematic functional block diagram of an conventional OFDM receiver without an ICI canceling element.
Referring to FIG. 1, a conventional OFDM receiver 10 may include a down-converter 1 down-converting a signal, which was up-converted by an OFDM transmitter (not shown), an analog/digital converter (ADC) 2 converting a signal output from the down-converter 1 to a digital signal, a guard interval (GI) remover 3 removing an inputted guard interval from the OFDM transmitter (not shown) for channel estimation in the signal output from the ADC 2, a Fast Fourrier Transform (FFT) unit 4 fast Fourier transforming a signal output from the GI remover 3, a channel estimation apparatus 6 estimating a channel frequency response based on a FFT signal converted by the FFT unit 4, an equalizer 5 equalizing the FFT converted signal based on the channel frequency response output from the channel estimation apparatus, and a decoder 7 decoding an equalized signal output from the equalizer 5.
As illustrated in FIG. 1, the equalizer 5 may be used in a static environment; however, it may have a remarkably lowered performance in a mobile environment because of not having a circuit for inter-carrier interference ICI cancellation.
A method for reducing the influence of ICI uses an OFDM symbol of a shorter duration. However, this method is weak due to a channel delay-spread and can not be used in single frequency networks.
ICI cancellation methods may be broadly divided into two kinds. One is a linear minimum mean square error LMMSE method and another is a decision-feedback error DFE method.
The LMMSE method has strength in canceling ICI almost perfectly; however, it requires high complexity. On the other hand, the DFE method has good performance with a comparably low complexity by estimating an ICI element through a preliminary decision or a hard-decision and performing repetitively a method to cancel an estimated ICI element from a received signal. However, in the DFE method, when an error percentage of the first preliminary decision or the hard decision is comparably big, there is an error propagation effect lowering the performance of total OFDM receiver as the DFE is performed repetitively. Therefore, a method and an apparatus for ICI cancellation, which can overcome the demerits of the aforementioned ICI cancellation methods and make the best use of each strength is desired.